Two points in the manufacture of nitro-glycerine are of the greatest importance, viz., the purity of the glycerine used, and the strength and purity of the acids used in the nitration.  With regard to the first of these, great care should be taken, and a complete analysis and thorough examination, including a preliminary experimental nitration, should always be instituted.  As regards the second, the sulphuric acid should not only be strong (96 per cent.), but as free from impurities as possible.  With the nitric acid, which is generally made at the explosive works where it is used, care must be taken that it is as strong as possible (97 per cent. and upwards).  This can easily be obtained if the plant designed by Mr Oscar Guttmann[A] is used.  Having worked Mr Guttmann’s plant for some time, I can testify as to its value and efficiency.

[Footnote A:  “The Manufacture of Nitric Acid,” Jour.  Soc.  Chem.  Ind., March 1893.]

Another form of nitric acid plant, which promises to be of considerable service to the manufacturer of nitric acid for the purpose of nitrating, is the invention of the late Mr Manning Prentice, of Stowmarket.  Through the kindness of Mr Prentice, I visited his works to see the plant in operation.  It consists of a still, divided into compartments or chambers in such a manner that the fluid may pass continuously from one to the other.  The nitric acid being continuously separated by distillation, the contents of each division vary—­the first containing the full proportion of nitric acid, and each succeeding one less of the nitric acid, until from the overflow of the last one the bisulphate of soda flows away without any nitric acid.  The nitrate of soda is placed in weighed quantities in the hopper, whence it passes to the feeder.  The feeder is a miniature horizontal pug-mill, which receives the streams of sulphuric acid and of nitrate, and after thoroughly mixing them, delivers them into the still, where, under the influence of heat, they rapidly become a homogeneous liquid, from which nitric acid continuously distils.

Mr Prentice says:  “I may point out that while the ordinary process of making nitric acid is one of fractional distillation by time, mine is fractional distillation by space.”  “Instead of the operation being always at the same point of space, but differing by the successive points of time, I arrange for the differences to take place at different points of space, and these differences exist at one and the same points of time.”  It is possible with this plant to produce the full product of nitric acid of a gravity of 1.500, or to obtain the acid of varying strengths from the different still-heads.  One of these stills, capable of producing about 4 tons of nitric acid per week, weighs less than 2 tons.  It is claimed that there is by their use a saving of more than two-thirds in fuel, and four-fifths in condensing plant.  Further particulars and illustrations will be found in Mr Prentice’s paper (Journal of the Society of Chemical Industry, 1894, p. 323).